An evaporator of a HVAC system typically has a tube bundle in a shell design. The tube bundle typically includes a plurality of tubes configured to carrya process fluid, such as water. Refrigerant in the shell can exchange heat with the process fluid in the tube bundle to cool down the process fluid.
Typically, the heat exchanging tubes extend a full length of the evaporator. In a single-pass evaporator design, a process fluid typically flows into inlets of the heat exchanging tubes from one end of the evaporator, through the full length of the evaporator, then out of outlets of the heat exchanging tubes from the other end of the evaporator. In a two pass evaporator design, the process fluid typically flows into inlets of first pass heat exchanging tubes from one end of the evaporator. The process fluid flows the full length of the evaporator through the first pass heat exchanging tubes, makes a turn in a returning box positioned at the other end of the evaporator, then flows into second pass heat exchanging tubes to return to outlets located at the same end as the inlets. Some evaporators may also have a four-pass design.
A water head is a device positioned on the end of the evaporator, and configured to distribute or receive the process fluid from the tube bundle. For example, in some evaporators, the inlets of the first pass heat exchanging tubes are positioned at a bottom section of the end of the evaporator, while the outlets of the second pass heat exchanging tubes are positioned at a top section of the end of the evaporator. The water head may have a fluid entrance and a fluid exit that have an “under-over” design, i.e., the fluid entrance is positioned at a bottom section of the water head and is configured to be in fluid communication with the inlets of the first pass heat exchanging tubes; and the fluid exit is positioned at a top section of the water head and is configured to be in fluid communication with the outlets of the second pass heat exchanging tubes.
In some other evaporators, the inlets of the first pass heat exchanging tubes may be positioned toward a left (or right) side of the evaporator, while the outlets of the second pass heat exchanging tubes may be positioned toward a right (or left) side of the evaporator. Accordingly, a water entrance and the water exit of a water head may be configured to have a “side-by-side” configuration.